At times, this specification recommends good practice
for authors and user agents. These recommendations are not normative
and conformance with this specification does not depend on their
realization. These recommendations contain the expression "We
recommend ...", "This specification recommends ...", or some similar
wording.

The fact that a feature is marked as deprecated (namely the 'aural' keyword) or going to
be deprecated in CSS3 (namely the system colors) also has no influence
on conformance. (For example, 'aural' is marked as non-normative, so
UAs do not need to support it; the system colors are normative, so UAs
must support them.)

All sections of this specification, including appendices, are
normative unless otherwise noted.

The validity of a style sheet depends on the level of CSS
used for the style sheet. All valid CSS1 style sheets are valid CSS 2.1
style sheets, but some changes from CSS1 mean that
a few CSS1 style sheets will have slightly different semantics in
CSS 2.1. Some features in CSS2 are not part of CSS 2.1, so not all CSS2
style sheets are valid CSS 2.1 style sheets.

A valid CSS 2.1 style sheet must be written according to the grammar of CSS 2.1. Furthermore, it must contain
only at-rules, property names, and property values defined in this
specification.
An illegal (invalid) at-rule,
property name, or property value is one that is not valid.

The document to which one or more style sheets apply. This is
encoded in some language that represents the document as a tree of elements. Each element consists of a name that
identifies the type of element, optionally a number of attributes, and a (possibly empty) content. For example, the source document could be
an XML or SGML instance.

The encoding language of the source document (e.g., HTML, XHTML, or
SVG). CSS is used to describe the presentation of document languages
and CSS does not change the underlying semantics of the document
languages.

(An SGML term, see [ISO8879].) The primary syntactic constructs
of the document language. Most CSS style sheet rules use the names of
these elements (such as P, TABLE, and OL in HTML) to specify
how the elements should be rendered.

An element whose content is outside the scope of the CSS
formatting model, such as an image, embedded document, or applet. For
example, the content of the HTML IMG element is often replaced by the
image that its "src" attribute designates. Replaced elements often
have intrinsic dimensions: an intrinsic width, an intrinsic height,
and an intrinsic ratio. For example, a bitmap image has an intrinsic
width and an intrinsic height specified in absolute units (from which
the intrinsic ratio can obviously be determined). On the other hand,
other documents may not have any intrinsic dimensions (for example, a
blank HTML document).

User agents may consider a replaced element to not have any
intrinsic dimensions if it is believed that those dimensions could
leak sensitive information to a third party. For example, if an HTML
document changed intrinsic size depending on the user's bank balance,
then the UA might want to act as if that resource had no intrinsic
dimensions.

The content of replaced elements is not considered in the CSS
rendering model.

The width and height as defined by the element itself, not imposed
by the surroundings. CSS does not define how the intrinsic dimensions
are found. In CSS 2.1 only replaced elements can come with
intrinsic dimensions.
For raster images without reliable resolution information, a size of
1 px unit per image source pixel must be assumed.

The content associated with an element in the source document.
Some elements have no content, in which case they are
called empty. The content
of an element may include text, and it may include a number of
sub-elements, in which case the element is called
the parent of those
sub-elements.

This term has two slightly different meanings in this
specification. First, a CSS parser must follow certain rules when it
discovers unknown or illegal syntax in a style sheet. The parser must
then ignore certain parts of the style sheets. The exact rules for
which parts must be ignored are described in these sections (Declarations and properties,Rules for handling parsing
errors,Unsupported
Values) or may be explained in the text where the term "ignore"
appears. Second, a user agent may (and, in some cases must) disregard
certain properties or values in the style sheet, even if the syntax is
legal. For example, table-column elements cannot affect the font of the
column, so the font properties must be ignored.

The content of an element after the rendering that applies to it
according to the relevant style sheets has been applied. How a
replaced element's content is rendered is not defined by this
specification. Rendered content may also be
alternate text for an element (e.g., the value of the XHTML "alt"
attribute), and may include items inserted implicitly or explicitly by
the style sheet, such as bullets, numbering, etc.

An element A is called a sibling of an element B, if and only if B
and A share the same parent element. Element A is a preceding sibling
if it comes before B in the document tree. Element B is a following
sibling if it comes after A in the document tree.

A user is a person who interacts with a user agent to view, hear, or
otherwise use a document and its associated style sheet. The user
may provide a personal style sheet that encodes personal
preferences.

A user
agent is any program that interprets a document written in
the document language and applies associated style sheets according
to the terms of this specification. A user agent may display a
document, read it aloud, cause it to be printed, convert it
to another format, etc.

An HTML user agent is one that supports one or more of the HTML
specifications. A user agent that supports XHTML [XHTML], but not
HTML is not considered an
HTML user agent for the purpose of conformance with this
specification.

CSS defines a finite set of parameters, called properties, that
direct the rendering of a document. Each property has a name (e.g.,
'color', 'font', or border') and a value (e.g., 'red', '12pt Times', or
'dotted'). Properties are attached to various parts of the document
and to the page on which the document is to be displayed by the
mechanisms of specificity, cascading, and inheritance (see the chapter
on Assigning property values, Cascading, and
Inheritance).

According to the definition of HTML 4, HEAD elements will be
inferred during parsing and become part of the document tree even if
the "head" tags are not in the document source. Similarly, the parser
knows where the P and LI elements end, even though there are no
</p> and </li> tags in the source.

Documents written in XHTML (and other XML-based languages) behave
differently: there are no inferred elements and all elements must have
end tags.

This section defines conformance with
the CSS 2.1
specification only. There may be other levels of CSS in the future
that may require a user agent to implement a different set of features
in order to conform.

In general, the following points must be observed by a user agent
claiming conformance to this specification:

For each source document, it must attempt to retrieve all
associated style sheets that are appropriate for the recognized media
types. If it cannot retrieve all associated style sheets (for instance,
because of network errors), it must display the document using those
it can retrieve.

It must parse the style sheets according to this specification.
In particular, it must recognize all at-rules, blocks, declarations,
and selectors (see the grammar of CSS 2.1).
If a user agent encounters a property that applies for a supported
media type, the user agent must parse the value according to the property
definition. This means that the user agent must accept all valid
values and must
ignore declarations with
invalid values. User
agents must ignore
rules that apply to unsupported media
types.

A user agent that renders a document with associated style
sheets must respect points 1-6 and render the document
according to the media-specific requirements set forth in this
specification. Values
may be approximated when required by the user agent.

The inability of a user agent to implement part of this
specification due to the limitations of a particular device (e.g., a
user agent cannot render colors on a monochrome monitor or page) does
not imply non-conformance.

UAs must allow users to specify a file that contains the user style
sheet. UAs that run on devices without any means of writing or
specifying files are exempted from this requirement. Additionally, UAs
may offer other means to specify user preferences, for example, through
a GUI.

CSS 2.1 does not define which properties apply to form controls and
frames, or how CSS can be used to style them. User agents may apply CSS
properties to these elements. Authors are recommended to treat such
support as experimental. A future level of CSS may specify this further.

CSS style sheets that exist in separate files are sent over the
Internet as a sequence of bytes accompanied by encoding
information. The structure of the
transmission, termed a message
entity, is defined by RFC 2045 and RFC 2616 (see
[RFC2045] and [RFC2616]). A message entity with a content type of
"text/css" represents an independent CSS document. The "text/css"
content type has been registered by RFC 2318 ([RFC2318]).